(1) Field of the Invention
This invention relates generally to polyolefin production. In particular, this invention relates to a process and a catalyst for making high melt index, low water vapor transmission, film grade, high density homopolymers from ethylene and copolymers from ethylene and another olefin selected from the group consisting of the C.sub.3 to C.sub.8 mono 1-olefins. The invention concerns a chromium-titanium silica-supported catalyst in which the high porosity silica support is prepared from an ammonia-stabilized silica sol which contains little sodium and which has an average silicate particle size ranging from about 2 to about 6 nm preferably about 5 nm.
(2) Description of the Related Art
It is known that chromium-titanium silica-supported catalysts can be used in polyolefin production. Numerous patents cover processes and catalysts in which the silica support is in the form of a hydrogel and is prepared using sodium silicate ("water glass") or other alkali metal silicates. U.S. Pat. No. 3,862,104 discloses formation of the hydrogel by the addition of an alkali metal silicate to an ammonium salt solution. The resultant gel is subsequently washed to reduce the alkali metal content. In U.S. Pat. Nos. 3,900,457, 3,870,656, and 3,887,494, the hydrogel is instead formed by adding an alkali metal silicate to an acid. After aging, the gel is also washed to reduce the alkali metal content and then is mixed with an oxygen-containing organic compound. Both the organic compound and water are then removed from the gel to form a substantially water-free support, a xerogel. The metals chromium and titanium may be added to the hydrogel either before gel formation (U.S. Pat. Nos. 3,870,656, 3,887,494, 4,436,882, 4,436,883) or after gel formation (U.S. Pat. Nos. 3,862,104, 3,900,457). A discussion of the controlling the rate of addition of silicate to the acid appears in U.S. Pat. Nos. 3,867,306 and 3,900,457.
Another patent, U.S. Pat. No. 4,547,557, discloses the preparation of a silica-titania cogel which does not contain alkali metal and thus does not require a washing step to remove alkali metal ions. The gel is prepared by partially hydrolyzing a silicate ester in an organic solvent and combining the partially hydrolyzed ester with a titanium tetraalkoxide to complete the hydrolysis. Addition of a base completes formation of the gel. A chromium compound may also be added to form a tergel, a three-component gel with all three components present in the mixture before gelation. Since an organic solvent rather than an aqueous solvent is utilized, azeotropic distillation with an organic liquid to remove water from the gel is not necessary.
The use of azeotropic distillation to remove water from the gel network without destroying the pore structure in the process is also known. See, e.g., U.S. Pat. Nos. 2,765,242, 3,862,104 and 4,081,407. A method of protecting the gel from sintering during the drying step is called surface esterification. The gel is refluxed in an alcohol to effect the reaction between the alcohol and the surface silanol groups on the gel. This method was first taught in U.S. Pat. No. 2,657,149 and first applied as a protection of silica gel in U.S. Pat. No. 2,765,242.